In a UML diagram where one class is comprised by another class, is there an implicit association table, or does it need to be shown?
For example, if I have Chapters that are composition of Paragraphs, is this alone enough:
Or does the association class to be explicitly shown like so:
I've never seen it done like that latter example, so I'm assuming it's implied. Or maybe I haven't normalized the data properly (considering chapter both appears in the Chapter class and the Paragraph class).
Simply, there is no implication. If you need an association class, you need to define it. However, UML is not about diagraming, it's about modeling. You can omit your association class in a diagram if you want to. The association class will still exist in your model, though.
No, it is not implied.
The reason why you haven't seen this is that in most cases it simply doesn't make sense. In the association class the class part additionally describes the properties of the association. In your example you create an artificial object that actually brings little or nothing. This kind of approach can be useful for many-to-many relationships which are impossible for composition (each part can have at most one whole). Even if you wanted to show Foreign Key it would simply be on one of the classes. But that's implied by the association itself. According to UML specification an inline attribute is equivalent to an association.
Moreover on UML you can depict many to many relationship simply but applying appropriate multiplicities on association ends. You may want to show the class depicting that only when modelling on the code level.
No, it is not implied.
UML is not about tables, but about classes. The author of the class diagram should tell the audience how a class should be interpreted. Some UML diagrams model the real world, others model a piece of application functionality and yet others depict a physical implementation.
Apparently, reading your question, you are modeling a relational database, where a class is a physical table. In that case, I would expect that every table is explicitly modeled as a UML class.
The UML standard does not demand this.
By the way, the notation (PK) and (FK) is not in accordance with the UML standard.
Related
I have recently been studying UML and drawing simple diagrams with ordinary plain arrows between classes, but I know it's not enough. There are plenty of other arrows: generalization, realisation and etc. which have meaning to the diagram reader.
Is there a nice resource which could explain each arrow (ordinary, plain, dotted, diamond-filled, diamond)?
It would be the best if it will have some code examples for them.
Here's some explanations from the Visual Studio 2015 docs:
UML Class Diagrams: Reference: https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2015/modeling/uml-class-diagrams-reference
5: Association: A relationship between the members of two classifiers.
5a: Aggregation: An association representing a shared ownership relationship. The
Aggregation property of the owner role is set to Shared.
5b: Composition: An association representing a whole-part relationship. The Aggregation
property of the owner role is set to Composite.
9: Generalization: The specific classifier inherits part of its definition from the general
classifier. The general classifier is at the arrow end of the connector. Attributes, associations, and
operations are inherited by the specific classifier. Use the Inheritance tool to create a
generalization between two classifiers.
13: Import: A relationship between packages, indicating that one
package includes all the definitions of another.
14: Dependency: The definition or implementation of the dependent classifier might change if
the classifier at the arrowhead end is changed.
15: Realization: The class implements the operations and attributes defined by the interface.
Use the Inheritance tool to create a realization between a class and an interface.
16: Realization: An alternative presentation of the same relationship. The label on the
lollipop symbol identifies the interface.
UML Class Diagrams: Guidelines: http://msdn.microsoft.com/library/dd409416%28VS.140%29.aspx
Properties of an Association
Aggregation: This appears as a diamond shape at one end of the connector. You can use it to
indicate that instances at the aggregating role own or contain instances of the other.
Is Navigable: If true for only one role, an arrow appears in the navigable direction. You can use
this to indicate navigability of links and database relations in the software.
Generalization: Generalization means that the specializing or derived type inherits attributes,
operations, and associations of the general or base type. The general type appears at the arrowhead
end of the relationship.
Realization: Realization means that a class implements the attributes and operations specified by
the interface. The interface is at the arrow end of the connector.
Let me know if you have more questions.
I think these pictures are understandable.
A nice cheat sheet (http://loufranco.com/wp-content/uploads/2012/11/cheatsheet.pdf):
It covers:
Class Diagram
Sequence Diagram
Package Diagram
Object Diagram
Use Case Diagram
And provides a few samples.
My favourite UML "cheat sheet" is UML Distilled, by Martin Fowler. It's the only one of his books that I've read that I do recommend.
For quick reference along with clear concise examples, Allen Holub's UML Quick Reference is excellent:
http://www.holub.com/goodies/uml/
(There are quite a few specific examples of arrows and pointers in the first column of a table, with descriptions in the second column.)
The accepted answer being said, It is missing some explanations.
For example, what is the difference between a uni-directional and a bi-directional association? In the provided example, both do exist. ( Both '5's in the arrows)
If looking for a more complete answer and have more time, here is a thorough explanation.
A very easy to understand description is the documentation of yuml, with examples for class diagrams, use cases, and activities.
Aggregations and compositions are a little bit confusing. However, think like compositions are a stronger version of aggregation. What does that mean?
Let's take an example:
(Aggregation)
1. Take a classroom and students:
In this case, we try to analyze the relationship between them. A classroom has a relationship with students. That means classroom comprises of one or many students. Even if we remove the Classroom class, the Students class does not need to destroy, which means we can use Student class independently.
(Composition)
2. Take a look at pages and Book Class.
In this case, pages is a book, which means collections of pages makes the book. If we remove the book class, the whole Page class will be destroyed. That means we cannot use the class of the page independently.
If you are still unclear about this topic, watch out this short wonderful video, which has explained the aggregation more clearly.
https://www.youtube.com/watch?v=d5ecYmyFZW0
If you are more of a MOOC person, one free course that I'd recommend that teaches you all the in and outs of most UML diagrams is this one from Udacity: https://www.udacity.com/course/software-architecture-design--ud821
I found a definition for association in UML as below.
An "association" in UML is defined as a kind of relationship between
classes,which represents the semantic relationship between two or more classes that
involves connections (links) among their instances .
I am not clear what is semantic relationship. Can anyone explain it with example with comparing it with non semantic relationship?
Associations in plain text
An association is a semantic relationship. The UML clause means that there is a structural relationship between instances of the associated classes independently of any specific implementation. "Semantic" underlines that the relationship is between the instances themselves, and not just "accidentally" for an operation:
Use associations primarily where there are structural relationships among objects. Do not use them to show transient relationships such as parameters or local variables of procedures.- Booch, Rumbaugh & Jacobson in Unified Modeling Language User Guide, 2nd edition
More arguments
What is "semantic"?
The term "semantic" is borrowed from linguistics and refers to the meanings behind the words. Linguists and map words (e.g. "Car", "Driver") to their meaning (e.g. a real car, a real person), and analyze the relation between words with a view of this mapping:
So, applying this to UML (modelling language) if you'd have the classes Car and Driver, you CAN model the semantic relationship as an association between the two classes.
What is not semantic?
Not all relationships are of semantic nature. You can have dependencies, which can express a technical relationship:
a transient relationship during an operation: with a factory pattern for example, a DriverFactory would «create» a Driver instance. Both instances the factory and the driver are related only at the exact moment of the creation operation. But the two instances are completely independent the nanosecond after.The same applies if the implementation of an operation needs to create a local instance of another class. Both classes are not associated, since we could imagine another implementation that works without such an instance.
a structural dependency: Maybe an operation require some other classes as parameter. Since the parameters themselves are transient, there is no association. But nevertheless, teh class needs to know about the other class.
For the records, I'm grateful to this public domain contributor for the nice car and driver and to 18f for advice on inclusive communication.
The subject of Semantics is sense. If one thing has something that is connected by sense to another thing, it is the semantic relationship. That definition is terribly wide. And, applied to UML relationship, incorrect. It is incorrect for two reasons.
First, UML covers not only Class-Instance languages, such as Java or C++, but pure object languages with heredity created by Prototypes as well. And this second variant is not covered by your definition at all.
Second, in UML you can have class A connected to class B through some complex AssociationClass, that is shown by a box with arrows, not arrow only. And it still will be named an association and it IS a semantic relationship, too. But a the semantic relationship goes from A to B through two classes, it is still a semantic relationship, but it is not an association in UML.
If you are trying to be deep in subject, better read the UML documentation: "An Association classifies a set of tuples representing links between typed instances." (UML 2.5, pharagraph 11.5.1). Notice: ANY link between two instances can be shown as an association. Maybe the book you are reading is wholely not so bad, but in the very place that you have cited the author merely tries to use pretty words not understanding their meaning and not even trying to be understandable to readers and to be CORRECT.
A model says something about the things being modeled. This is its semantics. Almost all elements of UML have semantics, defined in the sections titled "semantics". One exception is the comment. Adding a comment to a model doesn't influence its meaning.
Then why does the UML say this explicitely about associations? The reason is, that an association may or may not say something about the structure. If it doesn't, it is purely semantic.
For example it could tell us, that a Car can have at most one Driver. It doesn't necessarily mean that the car has an attribute of type Driver.
If we want to model that the two associated classes own attributes typed by the other end of the association, the notation will show this by small dots on its ends. Many people are not familiar with this notation and interpret associations without a dot in the same way. However, without the dots attributes are owned by the association itself and the structure of the classes is not influenced.
By the way, a class being the type of a parameter of an operation or having a dependency also means a semantic relationship.
If I create a conceptual class diagram such that each class captures 'name' and 'attributes' but not 'operations', have I not basically created what would be otherwise considered an ERD? I'm trying to gain an understanding of what the differences are between creating a conceptual class diagram as I have described versus calling it a ERD? If these are still two different animals, can somebody please explain what the differences are?
The class diagram contains just the classes in your object model with eventual links/relationships connecting diagram elements. However those links don't necessarily correspond to physical relationships like in an ERD diagram, but instead they represent logical connections.
The class diagram is just the object model of your application and does not contain any persistence-specific information. When you think about the class diagram forget about the database or any other storage you may use.
The ERD diagram on the other side, is a persistence-specific diagram which display the entities (tables) existing in a (most often) relational database. It also displays the physical relations (and cardinalities) between those tables and all other database-specific information. The ERD diagram can sometimes look similar to the class diagram, but that doesn't mean is the same as a class diagram.
There´s little difference in the expressiveness of both (if we just focus on the attributes, classes and associations part) if you use Extended Entity Relationship diagrams (the most common case nowadays)
True, they look very different at the graphical level since they use different symbols for the elements but the "semantics" are quite similar. They both allow inheritance (again, I´m talking about EER), n-ary associations, association classes, ...
The ER diagrams I've seen (most frequently ERWin IE notation) have focused on the design for a database. They are concerned with primary keys, foreign keys, have unnamed relationships, and usually have no generalization / specialization.
A good UML conceptual class diagram, on the other hand, is not concerned with keys, reflects the problem domain, and has association-end properties that at least hint at the semantics of why things are related. This helps communicate the domain down to more junior developers so they don't have to guess.
It depends on the situation where you may not like to do the ER-D. But imagine if you have a seperate data layer where the data logic is handled. In this case many details of data shall not be shared with the application layer. And you class diagram shall not go beyond the application layer. I must stress that both the diagrams are not equal. And there are situations where you need to do both, mainly in multi-tier architecture, and there are situations where you may be able to just use class diagram; e.g. single-tier application.
I strongly advocate the view that class diagram doesn't abrogate the E-R diagram.
Design class diagrams are made from conceptual model and collaboration diagrams.
Design class diagrams include:
Classes, associations and attributes
Methods
Types of attributes
Navigability
Dependencies
IMO In Simple terms
Class diagram depicts the details of how will the system work.
ER diagram depicts how the system persists 'state' as a blue print.
Goal:
Detail out state and behavior of the components(classes) of the system.
Design 'efficient', flexile system(less coupling and more cohesion) using Solid principles.
Goal:
Design a blue print of how to 'efficiently' persist the state of the system.
Consider what kind of queries will be made (read vs write), are there any joins required
consequently figure out the columns for indexing
Use Normalization, ACID properties.
PS: notice the both the diagram tries to efficiently do thing in their on respect.
Sorry for this newb question, i'm new to UML.
The diagram for a system is this one:
From what I know of UML, none of the classes in this diagram can own instances of the associated class as there's no aggregate relationship with it.
Does this mean in an implementation of the system in Java, based on the diagram, an outside class has to own instances of the associated class?
Sorry if the answer is obvious. I've spent hours scratching my head over it.
First off, terminology. #Daniel is right, you don't have an association class. However, I don't think you mean Association Class:
Does this mean in an implementation of the system in Java, based on the diagram, an outside class has to own instances of the associated class?
If I understand correctly that's the nub of your question. In implementation terms, which class(es) have a member variable containing a list of references to instances of Associated Class?
Again - if I understand right - your question stems from the following logic:
In UML, "ownership" is commonly described as a quality of Aggregation (or Composition) relationships.
The relationship between Aggregated/Composite PART Class and Associated Class is a simple binary association - not Aggregate/Composite.
Therefore the "ownership" property doesn't apply
Therefore who owns the list of references to Associated Class instances?
If that's right then the issue is with the specific meaning of "ownership". Whilst not tightly defined in UML, "ownership" typically means responsibility for managing full lifecycle.
I think you're interpreting it more generally: that if an association isn't aggregate, then the participating classes can't hold references to each other.
That's not the case. It's perfectly reasonable for Aggregated/Composite PART Class to hold a reference (or list of references) to instances of Associated Class. The inverse is equally valid. In some cases both are valid (with the attendant need to maintain consistency).
So in summary: is it necessary for an outside class to own the instances of Associated Class? No. It's perfectly valid for either or both ends of a binary association to manage instances of the relationship.
hth and apologies if I misunderstood your question.
PS: a final observation: be very careful about what you mean when using Aggregation. It's notoriously imprecise in the UML spec. Composition has a more rigorous definition, and you can cover 99% plus of all modelling scenarios using Composition and plain Binary Associations. About the only place Aggregation has a well-defined meaning not completely covered by the other two is denoting when recursive relationships must be acyclic.
UML does not specify the full behaviour of a system. So what do you mean, when you say an object owns another object? Also instances AssociatedClass could be root objects that are not owned by any other object.
The diagram you provided doesn't really contain an association class. The class you named 'associated class' is just a normal class. It also isn't owned by anything (that we see in the diagram).
If what you had in mind was association class, then take a look at example diagram with association class:
In this example, the MilleageCredit is an association class. So for each distinct combination of Fligh-FrequentFlyer there is one MilleageCredit.
As for ownership, since the Association class represents a relation between 2 associated objects, it gets deleted when
the association is cleared
either or both of associated objects are deleted
So if you delete either the Flight or FrequentFlyer the MilleageCredit will be gone too.
Also if you unlink Flight from FrequentFlyer again the MilleageCredit will be delete.
There's plenty of good UML docs online, for example UML basics: The class diagram
Hope this helps, otherwise please provide more info in the question.
I have read several tutorials on what a UML model should contain and what not. As a developer I always think in terms of a relational data model where you could never have a many to many relationship between tables. Now with a UML class model, I've read that if they don't provide added value, you could just skip the linktables.
However I've also read a tutorial where examples where given using data inside tables and was strongly suggesting to picture each class as a simple table while modeling your class model.
I am confused, what is it now?!
The "relational link table" is an implementation technique for a Many-to-Many relationship.
The relationship exists irrespective of how it's implemented.
In an object model, you have many choices for how to implement many-to-many, which may (or may not) involve an additional persistent table. It could be done lots of different ways.
The point of UML is to be able to describe the essential features of what the model really is.
You can also describe the implementation. They're separate diagrams with separate purposes. You can easily show the relational implementation with the link table. You can also show the essential model without the link table.
Here's the essential relationship
Here's the implementation of the relationship
Both are valid UML. The real question is "what do you need to show other people?" Essential truth or one particular implementation?
Model it as an M:N relationship. Same as in a relationship model there will be an associate class (or link class) of some kind in code. No need to put that on the diagram unless it has attributes on top of the join attributes (much like you would generally omit a join entity from an ERD unless it had attributes that weren't foreign keys in the related entities). The link class is typically drawn as a class connected to the relationship by a dashed line.
The Enrolment join entity is a good example of this in UML 2 Class Diagrams.